Microscopic theory of ultrafast out-of-equilibrium magnon-phonon dynamics in insulators

The interaction between lattice and spins is at the heart of an extremely intriguing ultrafast dynamics in magnetic materials. In this paper, we formulate a general nonequilibrium theory that disentangles the complex interplay between phonons and magnons in a THz laser-excited antiferromagnetic insulator. The theory provides a quantitative description of the transient energy flow between the spin and lattice subsystems, subject to magnon-phonon and phonon-phonon scatterings, giving rise to finite lifetimes of the quasiparticles and to the equilibration time of the system. We predict a kind of scattering process where two magnons of opposite polarizations decay into a phonon, previously omitted in the literature. The theory is combined with first-principles calculations and then applied to simulate realistic dynamics of NiO. The main relaxation channels and hot spots in the reciprocal space, giving the strongest contribution to the energy transfer between phonons and magnons are identified. The diverse interaction strengths lead to distinct coupled dynamics of the lattice and spin systems and subsequently to different equilibration timescales.